Within a wireless network, some wireless non-access point stations (non-AP STAs) may include high performing stations (STAs) such as laptops, etc, which are less constrained by battery life, and smaller battery powered non-AP STAs such as for example sensors, which need to last many years with small battery capacity. Non-AP STAs typically enter a power save (PS) mode when not needing to send or receive frames. The PS mode is especially important for the non-AP STAs with smaller batteries. To set the times when a non-AP STA needs to be awake, the IEEE 802.11e standard uses a procedure called Unscheduled Automatic Power Save Delivery (U-APSD). U-APSD allows a STA to be awake during a Service Period (SP). An unscheduled SP may begin when an AP receives a trigger frame, in the form of a QoS data or QoS Null frame, from a non-AP STA. Each of the four access categories (AC's) defined in EDCA, corresponding to AC_VO, AC_VI, AC_BE and AC_BK, can be configured separately to be trigger/delivery-enabled. The configuration is communicated by a non-AP STA for example in an association or re-association frame. The AP will then start buffering frames mapping to trigger enabled AC's, and will deliver them upon receiving a QoS trigger from the non-AP STA. However, the non-AP STA must wake up every DTIM time period in order to listen for beacons from an AP to determine if any group-addressed frames have been buffered for it by the AP. Waking up at DTIM time intervals however can further shorten the non-AP STA's battery life, especially in the case of a non-AP STA with a small battery.